1. Field of the Invention
The present invention relates to electro-optic wave guides and more particularly to an electro-optic wave guide fabrication method for reducing the incidence of damage induced by light passing through the wave guide.
2. Prior Art
Scanning systems which utilize laser beams for transmitting information are known. U.S. Pat. No. 3,848,087, for example, entitled "Optical Scanner Control System" discloses a scanning system which utilizes a multifaceted mirror for delecting a light beam over a scanning area. As successive mirror facets rotate into the path of an incident light beam the beam repetitively sweeps across the scanning area. Systems which utilize apparatus such as that disclosed in the '087 patent have applications in both raster input scanning (RIS) and in raster output scanning (ROS). In raster input scanning, the light beam illuminates an original document line by line and enables the information contained in that document to be encoded through procedures known in the art.
According to the invention disclosed in U.S. Pat. No. 4,396,246 entitled "Integrated Electro-optic Wave Guide Modulator", granted Aug. 2, 1983, a single electro-optic wave guide substrate operating in line with a laser light source confines light energy from the source and both intensity modulates and deflects the energy as it passes through. Electrodes coupled to the substrate and also coupled to controlled sources of electrical energy provide the modulation and deflection by changing the light transmission characteristics of the substrate. The deflection capabilities of the substrate are such that after the modulated light beam leaves the substrate it can be made to track and remain substantially coincident with the center of a moving facet of a multi-sided mirror. This capability can be used to enhance the functioning of the raster output scanner in a manner previously achieved with more expensive and less efficient acousto-optic coupling devices.
The preferred wave guide substrate comprises a lithium niobate crystal having a thin wave guiding layer formed by controlled out diffusion of a chemical constituent of the lithium niobate. U.S. Pat. No. 4,071,396, for example, discloses a technique for out diffusing Li.sub.2 0 (lithium oxide) from a Li Nb 0.sub.3 (lithium niobate) crystal by maintaining the lithium niobate crystal in vapor phase equilibrium with a specially constructed crucible. Controlled out diffusion using the technique disclosed in the '396 patent increases the refractive index of a thin surface layer thereby creating a wave guide.
A concern with prior art electro-optic wave guides is the incidence of optical damage caused by the optically coupled laser beam. The operational life of a lithium niobate wave guide is related to the degree to which it can resist optical damage since the development of optical damage increases the wave guides' insertion loss. To compete with a prior art acousto-optic modulator, for example, the electro-optic wave guide must have a mean time between failure of 40,000 hours.